1. Field of the Invention
The present invention relates to surfaces used to reflect light, and particularly to highly light reflectant surfaces that provide even diffusion of light energy from their surfaces.
2. Description of Related Art
Special light reflectant surfaces are used in a variety of applications requiring light energy to be close to completely reflected while providing an even distribution of light from the surface. While good mirrored surfaces can provide nearly perfect reflectivity of visible light, the light energy exiting these surfaces does so only at an angle equal to the incident angle of light contact. For many applications it is important that light be reflected with an even distribution of light from the surface. This latter property is referred to as diffuse or "lambertian" reflectance.
The degree of diffuse light reflectivity is critical in many applications. For instance, displays used in electronic equipment (e.g., instrument panels, portable computer screens, liquid crystal displays (LCDs), etc.), whether relying on supplemental lights (e.g., backlight) or merely ambient light, require very good diffuse reflectant back surfaces to maximize image quality. Reflectivity is particularly critical with backlighted displays in battery powered equipment, where better reflectivity is directly related to smaller required light sources and resulting lower power demands.
One of the most demanding markets for reflective material is the portable or laptop computer display market. This market is demanding because it requires high levels of diffuse reflection from very thin materials. For example, it is very desirable in this market for the backlight reflector to be quite thin, i.e. less than 0.30 mm and preferably less than 0.15 mm, to minimize the thickness of the completed display assembly.
One of the materials commonly used as a diffuse reflector in laptop computer displays is a polyester film sold under the trade name of MELINEX.RTM. by ICI Films of Wilmington, Del. This material is available in very thin thicknesses, such as 0.15 to 0.3 mm. However, the maximum visible light reflectance of a 0.13 mm thick sheet of MELINEX polyester film is only about 88.4%. Furthermore, in measuring the diffuseness of this material, the diffusivity is much less than desired. Thicker films of this material, such as 0.25 mm thick material, achieves only a maximum visible reflectance of 94.3% while suffering from added thickness and undesirable stiffness.
In U.S. Pat. No. 5,596,450 to Hannon, et al., it is taught that exceptional diffuse light reflective properties can be achieved through use of an expanded polytetrafluoroethylene (PTFE) material that has a porous microstructure of polymeric nodes and fibrils. As is explained in that patent, sheets of expanded PTFE material have been demonstrated to have diffuse light reflective properties that exceed any other material presently known. For example, the Hannon et al. patent teaches that an expanded PTFE material with thicknesses above 1 mm will reflect over 98% of the visible light hitting its surface. A thinner material having a thickness of about 0.5 mm will still reflect about 97-98% of the visible light hitting its surface. While these light reflective properties are exceptionally high, this material does undergo significant decreases in diffuse light reflectivity as thicknesses decrease, particularly below 1 mm and especially below 0.5 mm.
Expanded PTFE material has been used in backlighted displays in the past. In U.S. patent application Ser. No. 08/520,633 to McGregor et al., which published as PCT Application WO 97/08571 on Mar. 6 1997, and issued as U.S. Pat. No. 5,838,406 on Nov. 17, 1998, it is taught that sheets of expanded PTFE can be used in backlit liquid crystal displays with excellent results. While that application teaches that expanded PTFE sheets can be created with a thickness range of 0.01 to 12.0 mm, it is taught that the thinner membranes should be layered into a thicker unit for use as a light reflective surface. The application tests sheets of expanded PTFE in a range of thicknesses of between 0.5 and 6.2 mm. While all of the tested materials demonstrated vastly improved diffuse reflectivity over previously available reflective materials, thinner material plainly suffered from diminished reflectivity. For instance, the 0.5 mm thick material demonstrated good reflectivity of about 97-98% across the visible spectrum of light (450-750 nm, but significantly less than those materials tested with thicknesses of 1.0 mm and above. There is no teaching in that application as to how to employ a material with very thin cross-section without substantial loss of light reflectivity.
Prior to the use of expanded PTFE material as a light reflector, the best known material was that described in U.S. Pat. No. 4,912,720 and sold under the trademark SPECTRALON by Labsphere, Inc., North Sutton, N.H. This material comprises lightly packed granules of polytetrafluoroethylene that has a void volume of about 30 to 50% and is sintered into a relatively hard cohesive block so as to maintain such void volume. Using the techniques taught by U.S. Pat. No. 4,912,720, it is asserted that exceptionally high diffuse visible light reflectance characteristics can be achieved with this material, with reflectance over previously available reflectant material increasing from 97% to better than 99% at a material thickness of at least 7 mm.
The reflectance of SPECTRALON also decreases significantly with decreasing thickness over most of the light spectrum. It is known that thin sections of SPECTRALON material, e.g., below 4 mm, may be doped with barium sulfate to help maintain light reflectance and diffuse properties. According to published data from the technical catalog of Labsphere, Inc., a 1 mm thick doped SPECTRALON material has a reflectance level of as high as 95.6% in the visible spectrum. It does not appear that SPECTRALON material is available in thin films of less than 1 mm thick, or very thin films of 0.30 mm or less, such as those desired for laptop displays. Furthermore at these thickness levels it is believed that adequate reflection performance can not be obtained with granulated PTFE material.
It should be evident that none of these previously described materials is particularly suitable to use in computer laptop displays since they all have significantly reduced reflectivity in thicknesses of less than 1 mm, much less the very thin 0.3 mm to 0.15 mm thicknesses of materials that are presently employed as computer laptop display reflectors.
Accordingly, it is a primary purpose of the present invention to provide a diffuse light reflective material that is both very thin and still has high light reflective properties.
It is a further purpose of the present invention to provide such a material that can be readily employed in backlit displays, such as those used in laptop computers and the like.